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Abstract:

The invention relates to a method for reducing polyhalogenated compounds
in incineration plants comprising at least one combustion chamber. The
aim of the invention is to provide a more efficient method. To achieve
this, SO2 is separated from the flue gas in at least one gas washer
and is recirculated to the combustion chamber.

Claims:

1. A method for reducing polyhalogenated compounds in incineration plants,
provided with at least one combustion chamber (1), characterized in that
sulfur dioxide SO2 is selectively separated from the flue gas in at
least one washer (5) and is re-circulated into the combustion chamber.

2. The method as defined in claim 1, said method comprising the following
processing steps:a) Separating out of hydrochloric acid in a first,
acid-gas washer (4) by adding water, as well asb) Separating out SO2
in a second, neutrally operated washer (5) by adding water and sodium
hydroxide and/or calcium hydroxide, wherein sodium sulfate and sodium
sulfite or calcium sulfate and calcium sulfite, dissolved in water, are
formed in the process;c) Combining the hydrochloric acid, the sodium
sulfate solution and the sodium sulfite solution and/or the calcium
sulfate solution and the calcium sulfite solution in a reactor (12),
wherein this results in a chemical reaction and the forming of sodium
chloride and sodium sulfate and/or calcium chloride and calcium sulfate,
as well as the release of sulfur dioxide gas;d) Feeding an inert carrier
gas into the reactor, as well ase) Guiding the sulfur dioxide-charged
carrier gas from the reactor to the combustion chamber (1) and
discharging the sodium chloride and the sodium sulfate and/or the calcium
chloride and calcium sulfate from the reactor.

3. The method as defined in claim 2, characterized in that the secondary
gas is combined with the carrier gas and the sulfur dioxide in the region
between the reactor (12) and the combustion chamber (1).

4. A device for reducing polyhalogenated compounds in incineration plants,
said device comprising at least one combustion chamber (1) havinga) A
first acid-gas washer (4) with a first line for discharging hydrochloric
acid (8), as well asb) A second, neutrally operated washer (5), arranged
downstream of the first washer, with a line (9) for feeding in sodium
hydroxide or calcium hydroxide and a second line (10) for discharging
sodium sulfite and sodium sulfate and/or calcium sulfite and calcium
sulfate;characterized in thatc) A reactor (12) is provided into which the
first and second line empties, along with a feed line for an inert
carrier gas (14), as well as a third line (13) for discharging sodium
chloride and sodium sulfate and/or calcium chloride and calcium sulfate
andd) A connecting line is provided between the reactor (12) and the
combustion chamber (1) for guiding the carrier gas and the sulfur dioxide
from the reactor to the combustion chamber.

5. The device according to claim 4, characterized in that the connecting
line is provided with a location for feeding in the secondary gas.

6. The device according to claim 5, characterized in that the feed-in
location empties in the direction of the combustion chamber into the
connecting line.

7. The device according to claim 4, characterized in that the connecting
line empties in the region of the waste gas burn-out zone into the
combustion chamber.

Description:

[0001]The invention relates to a method and a device for reducing
polyhalogenated compounds in incineration plants, provided with at least
one combustion chamber as defined in patent claim one and patent claim
four. The method and the device are furthermore suitable for lowering
boiler corrosion by reducing the Cl content of the ash deposits produced
during the plant operation.

[0002]Polyhalogenated compounds are, for example, poly-chlorinated
dibenzo-p-dioxins and dibenzofurans (PCDD/F) that form during combustion
processes, among other things also during the waste-material
incineration, and which are released along with the waste gas. As a
result of their toxicity, the legislature for the Federal Republic of
Germany in the 17th Federal Emissions Safety Regulation (17th
BImSchV) has established a limit value for emissions of these compounds
from waste incineration plants of 0.1 ng TEQ/Nm3 (TEQ--toxicity
equivalent). Based on the present level of knowledge, this limit value
for PCDD/F emissions from incineration plants cannot be met by simply
optimizing the conditions for incineration. Insofar, state of the art
teaches the use of an additional, secondary flue gas cleaning to lower
the PCDD/F concentration in the incineration waste gas below the
prescribed value. Thus, incineration plants essentially consist of a
combustion chamber, if applicable with a boiler, at least one subsequent
dust collector, as well as at least one wet washer.

[0003]It is generally known from references [1] and [2] that the PCDD and
the PCDF component in a waste gas can be reduced considerably by simply
using an excess amount of sulfur for the combustion. In particular the
ratio of sulfur dioxide to hydrochloric acid is important in that case,
wherein the PCDD and PCDF component in the waste gas is reduced
considerably with an increase in the ratio of sulfur dioxide to
hydrochloric acid.

[0004]A method for reducing the corrosion in incineration plants is
additionally disclosed in reference [3]. With this method, a partial
stream of the flue gas is guided back or re-circulated into the
combustion chamber, following the separating out of most of the
hydrochloric acid in a first, acid-gas washing stage, by mean of a nozzle
which serves to feed in a secondary gas. Owing to the low hydrochloric
acid content and because the sulfur dioxide content has not yet been
reduced in a second washing stage, the flue gas has a ratio of sulfur
dioxide to hydrochloric acid which exceeds 1. The share of hydrochloric
acid in the untreated waste gas inside the combustion chamber is
consequently reduced solely by diluting it with the re-circulated waste
gas.

[0005]However, extremely large amounts of re-circulated waste gas are
required to achieve a noticeable change in the S/Cl ratio, which results
in a considerable increase in the waste-gas volume flow in the area of
the boiler and the dust collector. If no further measures are taken, this
leads to a reduction in the combustion temperatures and thus to
efficiency losses, wherein these could be mitigated through an additional
heating of the gases fed into the combustion chamber or by adding
chemicals or auxiliary agents.

[0006]Starting with the above, it is the object of the present invention
to propose a device and a method for reducing polyhalogenated compounds
in incineration plants with at least one combustion chamber, for which
the aforementioned disadvantages or restrictions do not occur or only to
a negligible degree.

[0007]This object is solved with a method having the features as defined
in claim 1 and a device having the features as defined in claim 4. The
dependent claims contain advantageous embodiments.

[0008]An essential and fundamental idea upon which the invention is based
is that SO2 is selectively separated out of the flue gas in at least
one washer and is re-circulated back into the combustion chamber as
SO2 or sulfuric acid. As a result of the selective separating out of
SO2 in a washer, highly concentrated SO2 or sulfuric acid and,
in particular, not a diluted waste gas mixture with low SO2
concentrations is advantageously available for re-circulation, which
considerably reduces the aforementioned disadvantages according to prior
art resulting from the extreme diluting.

[0009]The invention is explained in further detail in the following with
the aid of an exemplary embodiment and the following FIGURE, wherein:

[0010]FIG. 1 shows a schematic representation of the process flows when
using the method.

[0011]FIG. 1 shows that in connection with an incineration plant, the
invention involves a combustion chamber 1 with an oxygen-containing
primary and secondary gas feed-in 6 and/or 16, a boiler 2, or a different
component part for cooling the untreated waste gas 7, as well as
downstream-connected cleaning stages for the untreated waste gas. These
cleaning stages comprise a dust collector 3, for example a woven filter,
a first acid-gas washer 4, as well as a second neutrally operated washer
5. After passing through the cleaning stages, the cleaned waste gas 11 is
vented to the outside, for example by way of a chimney. The invention
furthermore relates to a reactor 12 with a carrier gas 14 supply line, a
first and a second line 8 and/or 10 as connecting lines to the two
washers 4 and 5, as well as two discharge lines 13 and 15, wherein the
discharge line 15 is connected to the secondary feed line 16 for a
connection to the combustion chamber.

[0012]Within the framework of the processing flows, a combustible material
is initially burnt inside the combustion chamber 1 with a feed-in of
primary gas 6. This is followed by an afterburning of any components not
completely burnt, along with a feed-in of a secondary gas 16. The
resulting untreated waste gas 7, which has a temperature in the range of
1000° C., is guided toward a boiler wall where it releases a
certain amount of heat through heat transfer to the boiler and/or the
medium inside the boiler. In the process, the untreated waste gas is
cooled to temperatures ranging from 200 to 300° C. The untreated
waste gas subsequently flows through the first cleaning stage and then
the dust collector 3, which the waste gas for the exemplary embodiment
leaves at the same temperature level and in the direction of the first
washer 4.

[0013]Dioxins form primarily in the waste-gas line at a temperature level
above 200° C., meaning at precisely the aforementioned temperature
level for the boiler wall 2 and the dust collector 3. However, as
described before, they can be reduced effectively by feeding in sulfur
dioxide or even sulfuric acid, for example together with the secondary
gas.

[0014]In the first acid-gas washer, hydrochloric acid is selectively
separated out through absorption in water at pH values of less than 1,
without this resulting in a separating out of sulfur dioxide. In addition
to the water feed line 17, this washer is furthermore provided with a
first line 8 for discharging hydrochloric acid in the direction of the
reactor 12, as well as with a line to the second washer 5.

[0015]In contrast, the second and neutrally operated washer is used for
the selective separation of sulfur dioxide from the waste gas. In
addition to a feed line 9 for water-dissolved sodium hydroxide or calcium
hydroxide as neutralizing agent, it also comprises a second line 10 for
discharging a watery solution of sodium sulfate or sodium sulfite and/or
calcium sulfate and calcium sulfite to the reactor 12, as well as a line
for venting the cleaned waste gas 11.

[0016]The aforementioned sulfites are formed through reaction of sulfur
dioxide with the respective hydroxides, as shown below:

2NaOH+SO2→Na2SO3+H2O (1) and/or

Ca(HO)2+SO2→CaSO3+H2O (2),

wherein at pH values below 7 the sulfites can in part oxidize further with
the oxygen in the waste gas to form sulfates, as shown below:

Na2SO3+1/2O2→Na2SO4 (3) and/or

CaSO3+1/2O2→CaSO4 (4).

[0017]While the aforementioned sulfates are chemically stable and are
therefore not available for generating sulfur dioxide in the reactor 12,
the above-mentioned sulfites can enter in the reactor 12 into a reaction
with the hydrochloric acids, supplied by the first washer 4, forming
water and chlorides in the process, and can ultimately react to form
sulfur dioxide:

Na2SO3+2HCl→2NaCl+SO2(g)+H2O (5) and/or

CaSO3+2HCl→CaCl2+SO2(g)+H2O (6).

[0018]In the same way as the aforementioned sulfates, the chlorides are no
longer needed for the further cleaning process and are vented to the
outside of the reactor 12 via the discharge line 13.

[0019]It is therefore critically important to adjust the oxidation rate
and thus also the pH value in the second washer. The goal basically is to
achieve a selective separation of sulfur compounds with high sulfite
content and low sulfate component. If the pH value is noticeably below 7,
the sulfites are further oxidized to form the undesirable sulfates [1].
On the other hand, pH values rising above 7 lead to an undesirable
separation of carbon dioxide from the waste gas, resulting in the forming
of carbonates in the washing solution:

2NaOH+CO2→Na2CO3+H2O (7) and/or

Ca(OH)2+CO2→CaCO3+H2O (8).

[0020]In turn, this requires the additional use of neutralizing agent.
Furthermore, during the mixing with HCl in the reactor, the yield of
SO2 would drop drastically, which is undesirable, since the HCl
required for generating SO2 would be used up by the competing
reaction:

Na2CO3+2HCl→2NaCl+CO2(g)+H2O (9) and/or

CaCO3+2HCl→CaCl2+CO2(g)+H2O (10).

[0021]For the exemplary embodiment, a sulfur dioxide yield could be
achieved in the second washer for pH values between 4 and 9, wherein a
maximum yield could be observed for a pH value adjustment around 7.

[0022]Two previously mentioned substance flows leave the reactor. The
stable compounds sodium chloride and sodium sulfate and/or calcium
chloride or calcium sulfate, which cannot be utilized with the method
according to the invention, are discharged via the discharge line 13 for
a further disposal, for example to a disposal site. On the other hand,
the sulfur dioxide formed in the reactor during the reactions (5) and/or
(6) is re-circulated via the discharge line 15 to the combustion chamber,
together with the carrier gas that is fed into the reactor via the feed
line 14. It makes sense in this connection to combine the line 15 with
the secondary gas feed line 16, wherein combining the line 15 and the
secondary gas feed-in 16 can be configured such that a certain excess or
low pressure can be generated in one of the two lines by one of the two
substance flows, respectively in the other line, in order to aid the
transport and/or mixing.

[0023]For one practical application, the salt residues from the two
washers 4 and 5 are combined inside a small stirrer vessel, functioning
as reactor 12, and are mixed together to form a solution, wherein
nitrogen dispersed via the feed line 14 into the solution served as
non-reactive carrier gas. The sulfur dioxide gas, released during the
reaction according to equations (5) and (6), is separated out in the
reactor together with the carrier-gas flow and is fed to the secondary
gas feed line 16. Starting with the combustion chamber 1, the
concentration of sulfur dioxide in the waste gas, increased in this way,
again passes through all components until it reaches the second washer 5,
where the sulfur dioxide is once more separated out selectively.

[0024]The aforementioned cyclic process leads to a step-by-step increase
in the sulfur dioxide concentration in the untreated waste gas, precisely
in the previously mentioned areas of dioxin formation. Under ideal
conditions, the ratio of sulfur dioxide to hydrochloric acid in the
untreated waste gas emitted by household waste incineration plants can be
raised without auxiliary agents from a value of 0.1 to values of around
0.6, wherein an increased sulfur dioxide concentration causes a reduction
of the chlorine (Cl2) that is primarily responsible for the PCDD/F
formation.